Firing mechanism with time delay and metering system

An apparatus for selectively isolating a firing head associated with a perforating gun, comprising:

• a first pressure activated firing head;
• an igniter coupled to the first firing head, the igniter generating a shock wave when detonated by the first pressure activated firing head;
• a time delay module coupled to the igniter, the time delay module having a deflagration component and a high-order detonation component, the time delay module generating a pressure pulse using at least the high-order detonation component after being activated by the igniter;
• a metering module coupled to the time delay module, the metering module including:
• a housing having a bore, at least one opening exposed to a wellbore annulus,
• a piston disposed in the housing bore, the piston having at least one passage, the piston being axially displaced from a first position to a second position by the generated pressure pulse, the piston sealing the at least one opening of the housing in the first position and allowing fluid communication through the at least one opening of the housing in the second position;
• and
• a second pressure-activated firing head coupled to the metering module, the second firing head being in fluid communication with the housing bore and in pressure communication with the wellbore annulus when the piston is in the second position, the second pressure-activated firing head being activated by a pressure increase in the bore.

A method for selectively isolating a firing head associated with a perforating gun, comprising:

• forming a perforating tool by:
• coupling an igniter to a first firing head;
• coupling a time delay module to the igniter
• coupling a metering module to the time delay module, the metering module including:
• a housing having a bore and at least one opening, and
• a piston disposed in the housing bore, the piston having at least one passage and
• coupling a second firing head to the metering module, the second firing head being in fluid communication with the housing bore and only in pressure communication with a wellbore annulus when the piston is in the second position;
• conveying the perforating tool into a wellbore;
• activating the igniter using the first firing head;
• activating the time delay module using a shock wave generated by the activated igniter;
• generating a pressure pulse using the activated time delay module;
• using the generated pressure pulse to axially displace the piston from a first position to a second position by the generated pressure pulse, the piston sealing the at least one opening of the housing in the first position and allowing fluid communication through the at least one opening of the housing to the bore in the second position;
• and
• increasing a pressure in a wellbore annulus after the bore of the metering module is filled with a fluid.